XPost: rec.radio.shortwave   
   From: detritus@ix.netcom.com   
      
   Don Pearce wrote:   
      
   > On Sun, 13 Nov 2011 08:38:28 -0700, Lord Valve   
   >  wrote:   
   >   
   > >dave wrote:   
   > >   
   > >> On Sun, 13 Nov 2011 11:39:03 +0000, Geoffrey S. Mendelson wrote:   
   > >>   
   > >> >> It is much more important to know exactly how long and how well your   
   > >> >> satellite is going to work than to hope to get longer by using a   
   > >> >> technology that might last longer, but will more probably die   
   > >> >> unexpectedly when struck by a cosmic ray burst.   
   > >> >   
   > >> > Sometimes you can not predict how long a satellite will be used. A   
   > >> > friend of mine worked on a civilian satellite for a defense contractor   
   > >> > and just before the division was sold off, cleaned out any old documents   
   > >> > and files they had on it.   
   > >> >   
   > >> > Since the satellite he had worked on was way past its expected life (but   
   > >> > still in use), the contracts had long expired, the work was not   
   > >> > classified and a new improved one was due to be launched in a few days,   
   > >> > he was told to dump it all.   
   > >> >   
   > >> > A few days later, the booster exploded on the pad, and the replacement   
   > >> > was destroyed.   
   > >> >   
   > >> > The sattelite was kept running for many years, although there were no   
   > >> > documents on what to do or how it was built.   
   > >> >   
   > >> > Geoff.   
   > >>   
   > >> What good is a diagram if the unit is 24,000 miles in the air?   
   > >   
   > >It had better *not* be in the air...  ;-)   
   > >   
   > >Besides - I saw mention upthread of using the ambient   
   > >vacuum with just the tube elements, rather than a typical   
   > >evacuated glass (or other material) enclosure...is the   
   > >vacuum in geosynchronous orbit really hard enough?   
   > >It would seem to me that there are probably plenty of   
   > >gas molecules floating around at that height, even if   
   > >it would still qualify as a "soft" vacuum.  Anybody?   
   > >   
   > >Lord Valve   
   > >   
   > >   
   >   
   > For all sorts of other reasons, standard enclosed tubes are used. Main   
   > reasons are first to contain the electrons so other metalwork doesn't   
   > get involved, and second to maintain the correct physical positioning.   
   > The helix is of very fine tolerance in both pitch and positioning.   
   > Space is certainly hard enough, but the environment around a satellite   
   > is frequently not space, but a diffuse cloud of exhaust gas which   
   > would extinguish a TWT immediately.   
   >   
   > d   
      
   Ah. Good point!   
      
   Satellites do indeed need to use propellant of some sort   
   to keep in position; I didn't think of that at all.  And it   
   would seem that even if the ambient vacuum were   
   hard enough, conventional construction of the TWT   
   would be needed to keep contaminants out of it during   
   the satellite assembly process down on Terra firma.   
   But I must admit, the idea of using ambient vacuum   
   tickles my fancy a bit.  ;-)   
      
   Lord Valve   
      
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